286754 Economic Optimum Design and Control of the Monoisopropylamine Process
In this work, the steady state economic optimum design and control of a process producing monoisopropyl amine (MIPA) via isopropyl alcohol (IPA) amination is studied. The principal reactions are the MIPA formation main reaction (IPA + NH3 ® MIPA + H2O) and the di-isopropylamine (DIPA) formation side reaction (IPA + MIPA ® DIPA + H2O). The separation of MIPA, DIPA, H2O, unreacted IPA and NH3, by the presence of IPA-H2O and IPA-DIPA homogeneous azeotropes is complicated.
A process flowsheet consisting of a high conversion packed bed reactor, 2 distillation columns, a decanter and a stripper along with two liquid recycles is shown to be capable of producing 99.5% pure MIPA. A high conversion reactor must be used to ensure the reactor effluent has sufficiently small amount of IPA. Post NH3 recovery in the first column, the decanter then exploits ternary liquid-liquid phase split to obtain nearly pure water (some IPA) and DIPA (some water) phases. The water stream is stripped to recover IPA and nearly pure water is discharged. The DIPA rich stream along with the recovered IPA is recycled back to the reactor. The single pass reactor conversion is a crucial design variable affecting the feasibility of product separation as too much IPA (low conversion) causes the free reactor effluent composition to be outside the liquid-liquid phase envelope. Even as the economic optimum design attempts to minimize the reactor size to the extent that liquid-liquid phase split just occurs in the decanter. Process operability considerations dictate reactor size over design. With sufficient reactor overdesign the hard operating constraint at maximum achievable throughput are flooding limits for the two columns and not the decanter.
An effective plantwide regulatory control structure is synthesized for the overdesigned reactor flowsheet and shown to provide robust process regulations for large throughput changes. The case study provides an interesting example of the ever-present conflict between economic design and process operability.